std::vector causing unexpected memory allocation patterns in C++20 with custom allocator
I'm learning this framework and After trying multiple solutions online, I still can't figure this out. I'm experiencing unexpected memory allocation patterns when using `std::vector` with a custom allocator in C++20. I created a custom allocator that tracks memory allocations and deallocations for debugging purposes. However, when I push back elements into the vector, I see that the memory usage is significantly higher than expected, and I suspect it might be related to how `std::vector` handles resizing under the hood. Here's a simplified version of my custom allocator: ```cpp #include <iostream> #include <memory> #include <cstdlib> template <typename T> class DebugAllocator { public: using value_type = T; DebugAllocator() = default; template <typename U> DebugAllocator(const DebugAllocator<U>&) {} T* allocate(std::size_t n) { std::cout << "Allocating " << n << " elements" << std::endl; return static_cast<T*>(::operator new(n * sizeof(T))); } void deallocate(T* p, std::size_t) { std::cout << "Deallocating" << std::endl; ::operator delete(p); } }; template <typename T, typename U> bool operator==(const DebugAllocator<T>&, const DebugAllocator<U>&) { return true; } template <typename T, typename U> bool operator!=(const DebugAllocator<T>&, const DebugAllocator<U>&) { return false; } ``` And here's how I'm using it in my code: ```cpp #include <vector> int main() { std::vector<int, DebugAllocator<int>> vec; for (int i = 0; i < 1000; ++i) { vec.push_back(i); } return 0; } ``` When I run this, I see multiple calls to `allocate` that seem to exceed the number of elements being pushed back. I expected that when the vector resizes, it would allocate a larger block of memory, but the output indicates many small allocations instead. This is leading to memory fragmentation issues and higher memory consumption than anticipated. I've also tried using `reserve()` to pre-allocate memory before the loop, but it didn't seem to change the allocation pattern significantly. I've verified that my allocator is correctly allocating and deallocating memory, but I'm unsure if there's a specific behavior of `std::vector` in C++20 that I'm missing. Could anyone help clarify this? Is there a recommended practice to manage memory more efficiently in this scenario? My development environment is Ubuntu. This is part of a larger CLI tool I'm building. I'd really appreciate any guidance on this. The stack includes C++ and several other technologies. I'm using C++ 3.11 in this project. Any feedback is welcome!